A polyimide resin is a useful engineering plastic that has high thermal stability, high strength and high solvent resistance due to rigidity, resonance stabilization and firm chemical bond of the molecular chain thereof, and is being applied to a wide range of fields. A polyimide having crystallinity is further enhanced in the heat resistance, the strength and the chemical resistance thereof, and thus is expected for applications as alternatives of metals or the like.
The major factors for the excellent thermal stability of a polyimide include the high glass transition temperature and the high melting point thereof. In consideration of the long-term heat resistance, the properties thereof have a strong correlation to the glass transition temperature. A material exposed to a high temperature for a prolonged period of time, for example, surroundings of an automobile engine and a packing of a high-temperature reactor, is necessarily prevented from being deteriorated in mechanical strength at a retention temperature thereof. Accordingly, super engineering plastics including a polyimide are often used in the case where a high temperature exceeding 200° C. is applied. A resin having a low glass transition temperature, such as nylon, is inferior in long-term heat resistance to super engineering plastics (see NPL 1). The glass transition temperature is in a temperature range where a mechanical strength is largely decreased in both an amorphous resin and a crystalline resin, and beyond that temperature, even a crystalline resin that is reinforced with a filler or the like may not avoid deterioration of the mechanical strength. Taking as an example polyetheretherketone, which is a highly crystalline resin with a crystallization degree of approximately 30%, the resin that is reinforced with a filler or the like has a deflection temperature under load that is approximately immediately below the melting point, but the mechanical strength thereof is largely decreased around 153° C., the glass transition temperature.
A polyimide has a high glass transition temperature but generally has no melting point appearing in a temperature range lower than the decomposition temperature. Vespel (registered trademark), a highly heat-resistant resin, is molded at a high temperature and a high pressure for a prolonged period of time and the like conditions since it has no melting point below the decomposition temperature, and thus is necessarily expensive due to the difficulty in the molding process thereof (see PTL 1).
For improving the moldability, it is necessary to impart thermoplasticity (i.e., a melting point in a crystalline resin) to a polyimide at a temperature lower than the decomposition temperature. A thermoplastic polyimide may be subjected to injection molding and extrusion molding, has good handling property, has recyclability, and thus may be a material that is considerably useful in industrial-scale production. As described above, however, a polyimide generally has no melting point below the decomposition temperature, and it can be said that a crystalline thermoplastic polyimide, which is capable of being subjected to injection molding and extrusion molding, is a scarce resin in the market.
A commercially available crystalline thermoplastic polyimide capable of being subjected to injection molding or extrusion molding includes Aurum (registered trademark) (Mitsui Chemicals, Inc.) (see PTL 2). The material is a rigid wholly aromatic polyimide but succeeds to have a melting point, which is generally difficult to be observed, at a temperature lower than the decomposition temperature by introducing plural flexible ether bonds and meta structures into the structure. The material has a large number of flexible structures but has a high melting point (388° C.), which is peculiar to a polyimide, and in particular, a higher temperature exceeding 400° C. is required for molding the material (see NPL 2). Furthermore, the crystallization rate thereof is far smaller than an ordinary injection molding cycle, and it can be said that there are difficulties in restrictions on equipments and handling property.
For improving the moldability, there have been some attempts of decreasing a melting point of a polyimide, but in practice, decreasing the melting point also decreases the glass transition temperature, thereby extinguishing the high glass transition temperature, which is peculiar to a polyimide. There is an empirical rule that the melting point and the glass transition temperature have a certain distance (in general, there is often an approximate relationship, (glass transition temperature)/(melting point)=⅔ in terms of absolute temperature), and therefore decreasing the melting point for improving the moldability generally also decreases the glass transition temperature, which is one of the basic factors of the heat resistance.
There has been reported, for a wholly aromatic polyimide, a polyimide having a melting point and a glass transition temperature that are very close to each other, which breaks the aforementioned relationship (herein, a polyimide having simultaneously a melting point of 360° C. or less and a glass transition temperature of 200° C. or more is referred to as a polyimide having a low melting point and a high glass transition temperature). For example, Vladimir, et al. have reported that a polyimide having a particular structure has very close property values, i.e., a melting point of 320° C. and a glass transition temperature of 204° C. (see NPL 3). However, the polyimide shows reproducible crystallinity only under the special condition, i.e., in the presence of carbon nanotubes mixed therewith, and therefore it is difficult to recognize the polyimide as a crystalline resin. Mitsui Chemicals, Inc. has reported that a copolymer polyimide having two kinds of repeating unit structures shown by the following formula (a) has further close property values, i.e., a melting point of 281° C. and a glass transition temperature of 229° C., while the values vary depending on the composition (see PTL 3).

However, attempting to impart a melting point lower than the decomposition temperature to a wholly aromatic polyimide requires the use of highly special monomers that have low versatility and difficulty in synthesis. The synthesis methods of the monomers in PTL 3 result in necessity of many process steps, prolonged periods of time, and special raw materials (see PTL 4). It is necessarily difficult to decrease the melting point in the case where a rigid aromatic structure is incorporated into both the acid component and the diamine component of the polyimide, which originally has a rigid structure, and as a result, there is less possibility of practical mass production in the market. For a fact, only Aurum (registered trademark) mentioned above has been known as a wholly aromatic crystalline thermoplastic polyimide in the large market exceeding several tens of tons.
On the other hand, there have been many cases where a crystalline polyimide is obtained from monomers that have high versatility, i.e., that may be easily synthesized or available, which are confirmed in a semi-aromatic polyimide system using an aliphatic diamine (herein, the semi-aromatic polyimide is defined as a polyimide obtained from an aromatic tetracarboxylic acid compound and an aliphatic diamine compound).
In particular, on synthesizing a semi-aromatic polyimide with a straight-chain aliphatic diamine and an aromatic tetracarboxylic acid compound, the straight-chain aliphatic diamine moiety forms a soft segment, whereas the aromatic tetracarboxylic acid moiety forms a hard segment, which may result in high crystallinity in some cases (see PTL 3). For decreasing the melting point for improving the moldability in this system, it is necessary to extend the chain length of the soft segment moiety, i.e., the chain aliphatic moiety. There is generally a tendency that the melting point is decreased inversely proportional to the chain length (see NPL 4).
In the semi-aromatic polyimide system, however, the glass transition temperature is largely decreased along with the decrease of the melting point according to the aforementioned general rule, thereby extinguishing the high glass transition temperature, which is peculiar to the polyimide. In consideration of the chemical structure, the phenomenon may be caused by such a fact that the introduction of the flexible structure increases the degree of freedom of the molecular chain movement and activates the thermal motion of the molecules. Accordingly, the semi-aromatic polyimide using an aliphatic diamine having a practical thermal property is difficult to be distinguished from the other resins including nylons and esters, and thus has a low competitive ability in the market.